A01911 alarm in siemens 6RA80 DC Drive indicate that the clock cycle synchronous operation clock cycle failure. This means the global control telegram to synchronize the clock cycles has failed - in cyclic operation - for several DP clock cycles or has violated the time grid specified in the parameterizing telegram over several consecutive DP clock cycles.

1. Check the physical bus configuration (cable, connector, terminating resistor, shielding, etc.).
2. Check whether communication was briefly or permanently interrupted.
3. Check the bus and controller for utilization level (e.g. bus cycle time Tdp was set too short).